JPH0552904B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to an automatic range switching circuit applied to digital voltmeters and the like.

[Conventional technology]

The following is a conventional example of this type of automatic range switching circuit.
There are methods (a) and (b).

(b) Digital comparator method.

The data obtained by converting the input into a digital signal is compared with the setting data, a high or low signal is extracted, and the range of the preamplifier is switched based on that signal.

In this method, the switching level can be set accurately. However, each sample of A-D conversion (analog-to-digital conversion)
Since it is designed to switch between two ranges, A-
When the D conversion time is long, the time required for range switching becomes long. The digital comparator function has the advantage that it can be realized by using a microprocessor without adding any special hardware.

(b) Analog comparator method.

In order to compensate for the drawback (time) of the digital comparator method, this method uses an analog comparator to compare the analog signal (preamplifier output) before A-D conversion with a set value, and before performing A-D conversion. It is designed to switch ranges. Although the switching label is not as precise as the digital comparator method, it has the advantage of being faster. However, there is a drawback that the circuit configuration increases compared to the digital method.

[Problem that the invention seeks to solve]

The present invention was made in order to solve the above-mentioned problems required for conventional range switching, and its purpose is to realize the above-mentioned characteristics (high speed) of the analog system using a digital system. The purpose is to achieve faster switching without adding special hardware.

[Means for solving problems]

In order to achieve the above object, the present invention converts the analog input to be measured into a digital signal by modulating the analog input to be measured into a pulse width signal and digitally counting the pulse width modulated signal N times. In a circuit having an analog-to-digital converter, the result of digitally counting the pulse width modulation signal N1 times, which is sufficiently shorter than N times, is used as data for range switching.
Examples will be described below.

〔Example〕

FIG. 1 is a connection diagram of an embodiment of the range switching circuit according to the present invention. In Figure 1, Ex is the input to be measured, SW is the switch, SC is the signal conditioner including the range switching circuit, PWM is the pulse width modulation circuit, GC is the gate circuit, COU is the counter, PD is the pulse detection circuit, and μP is the It is a microprocessor (hereinafter referred to as a processor).

The analog input Ex to be measured applied via the switch SW is applied to the pulse width modulation circuit PWM via the signal conditioner SC, and is modulated into a pulse width signal (hereinafter referred to as a PWM signal). The gate circuit GC is controlled by a gate control signal output from the processor μP, and a PWM signal and a clock pulse fc are applied to its input terminal, and its output terminal is connected to the counter COU. Counter COU sends count values D1 and D2, which will be described later, to processor μP. Pulse width modulation circuit PWM
A by gate circuit GC and counter COU
- A D converter is configured. The timing of the pulse detection circuit PD is controlled by the output of the processor μP, and is connected between the pulse width modulation circuit PWM and the processor μP. Range switching in the signal conditioner SC is performed by a range selection signal output from the processor μP. The operation of the automatic range switching circuit having such a configuration will be explained as follows with reference to FIG.

Measured input Ex applied by switch SW
is applied to the pulse width modulation circuit PWM via the signal conditioner SC, and its period is modulated into a PWM signal represented by Tc. This PWM signal is shown in FIG. Here, when an A-D start command signal is given to the A-D converter from the processor μP at time t in FIG.
PWM signal pulse width T1 period clock pulse fc
passes through the gate circuit GC, and the passed clock pulse fc is counted by the counter COU.
The above counting operation is performed every time the PWM signal is applied to the gate circuit GC. Normally, the input under test
Ex A-D conversion is performed by performing the counting operation of the above PWM signal N times (H is an integer) and integrating the N times (the data of the N times is assumed to be D1 + D2). At the time when N1 PWM signals, which are sufficiently smaller than the count (T=N・Tc), have been counted (T'=
The counting result at N1・Tc) (the counting result is designated as D1) is set to the setting value DH (High) in the processor μP.
DL (Low), and the processor μP keeps the range selection signal as a signal until DL<D1<DH.
Sends to conditioner SC to switch range. However, since the optimal range is detected using the data of N1/N, the set values DH and DL use data with corresponding weights. Note that a waiting time required for a PWM signal response is provided for each range change. The processor μP determines that the count value D1 falls within the range DL<D1<DH. When the optimum range setting is completed in this way, the counter COU performs an integral count for N cycles of the PWM signal. This changes the input to be measured Ex from A to D.
Converted data D1+D2 is obtained. that data
D1+D2 is given to the processor μP, and its value is displayed on a display device (not shown) under the control of this μP.

Note that when the value of the input to be measured Ex is sufficiently large for the currently set range, the pulse width modulation circuit PWM will no longer generate a pulse output. Therefore, in the circuit shown in Figure 1, the pulse width modulation circuit
A pulse detection circuit PD is provided to detect the presence or absence of a PWM output pulse. The pulse detection circuit PD detects the presence or absence of the output pulse of the pulse width modulation circuit PWM, and if there is no PWM pulse output, D1
>As in the case of DH, processor μP issues a range switching command to signal conditioner SC.
The appropriate timing for detecting the presence or absence of the PWM pulse output (based on the timing signal from the processor μP) is about 2Tc from the A-D conversion star signal.

To perform automatic zero point correction in the circuit shown in Figure 1, it is necessary to adjust the A-D conversion time of the range setting and the input to be measured.
Ei = 0 for each cumulative A-D conversion time of Ex,
All you have to do is find the result of Ei=Ex and find the difference between them.

〔Effect of the invention〕

As explained above, in the present invention, when the analog input to be measured is A-D converted by integrating N pulse width modulation signals, the
Since the configuration is configured to obtain A-D conversion data for changing the range in N1 times, it is possible to obtain an automatic range switching circuit that changes ranges faster than the conventional example in (a) above, although the resolution of range switching decreases. .

As an example, with a digital multimeter with 5 1/2 digit resolution (maximum display 199999), integration time T = 200ms,
When Tc=1ms.N=200, the A-D conversion time (for one sample) including automatic zero point correction is approximately 0.4 seconds. The resistance measurement range of a digital multimeter ranges from terminals open (100MΩ range) to terminals shorted (100MΩ range).
In the conventional method (a), it takes approximately 2.8 seconds (7 samples) to switch from 7 ranges to Ω range). On the other hand, in the circuit of the present invention, for example, the A-D conversion time for range determination is 1/10.
If you shorten it to ~1/100 (however, the resolution for determining the range switching level will drop to 4 1/2 to 3 1/2 digits, approximately 1 sample rate (= 0.4 seconds), and range switching will also be possible. It becomes possible to perform conversions including

[Brief explanation of drawings]

FIG. 1 is a connection diagram showing an embodiment of the automatic range switching circuit according to the present invention, and FIG. 2 is a waveform diagram for explaining the operation of the circuit shown in FIG. Ex...Input under test, PWM...Pulse width modulation circuit, GC...Gate circuit, COU...Counter, μP
...Microprocessor, PD...Pulse detection circuit.

Claims (1)

[Claims] 1. An analog input to be measured is applied to a pulse width modulation circuit via a range switching circuit to be modulated into a pulse width signal, and the pulse width modulation signal is applied via a gate circuit to which a clock pulse is applied. In a circuit having an analog-to-digital converter that converts an analog input to be measured into a digital signal by digitally counting N times (N is an integer) using a given counter, the count value of the counter As well as being taken in,
A microprocessor is provided in which high-side and low-side setting values of the range switching circuit are set, and the microprocessor uses a pulse width modulation signal that is sufficiently shorter than the N times counted by the counter.
The pulse width modulation signal is counted N1 times, and the N1 counted value is compared with the set value in the microprocessor, and the N1 counted value is within the high side and low side set values set in the microprocessor. an automatic range switching circuit, wherein the microprocessor is configured to provide a range selection signal to the range switching circuit;